More kids were hospitalized for serious injuries resulting from abuse in 2009 than in 1997, according to a new study - despite previous research suggesting fewer kids are maltreated now than in the past.

Researchers found the increase in serious injuries was due to more babies under a year old being hospitalized. The number of abused, injured kids and teenagers held relatively steady.

The findings are based on a survey of records from 2,500 or more U.S. hospitals conducted every three years.

"We're looking at the most seriously injured kids," said lead author Dr. John Leventhal, a pediatrician from the Yale School of Medicine in New Haven, Connecticut.

"The kids who get hospitalized for physical abuse represent a very small proportion of all the children in the country who are physically abused," he told Reuters Health - about two to four percent.

Leventhal and his co-author Julie Gaither consulted kids' hospital discharge forms for records of serious abuse, physical assault and shaken baby syndrome. They found the number of children hospitalized for those reasons increased slightly between 1997 and 2009, from 6.1 to 6.4 out of every 100,000.

Those children were kept in the hospital for a week, on average. Their most common injuries were fractures, open wounds and traumatic brain injuries.

Babies are known to be at the highest risk of physical abuse. Based on this sample, the number of infants under one year old with a serious abuse-related injury increased from 56.2 to 62.3 per 100,000 each year.

"Infants tend to be hospitalized at a much higher rate than older children, and I think it's because the injuries they sustain are much more serious," Leventhal said.

For example, he explained, slapping a six-year-old may leave a bruise, but slapping a baby could cause internal injuries.

And sleep-deprived parents are more likely to "lose it" with a baby who won't stop crying.

During the same time period, the number of babies, kids and teens hospitalized for serious injuries not related to abuse dropped by more than 20 percent, the researchers reported Monday in Pediatrics.

Leventhal said there are a few possible explanations for why serious injuries due to physical abuse may have increased while other studies have reported that abuse confirmed by Child Protective Services is on the decline.

It may be harder for CPS to substantiate abuse claims than in the past, he said. Or, most parents may be doing a better job of not hurting their children, but caregivers on the extreme end of the spectrum are still getting violent. Finally, it's possible that economic stressors could be causing more parents to lash out, according to Leventhal.

In another recent study, researchers found that kids' hospital admissions for physical abuse tended to increase as a local area's rate of delinquent mortgages and housing foreclosures rose (see Reuters Health story of July 17, 2012).

Dr. Joanne Wood, the lead author of that report from the Children's Hospital of Philadelphia, said the increase in hospitalizations in the new study is consistent with her own findings.

The results, she told Reuters Health, point to the importance of "doing a better job at the community level of identifying families who are under stress, who are under pressure, and helping to link them to resources that can help support them."

Leventhal said doctors need to work extra hard at catching the early signs of child abuse - as well as teaching parents how to deal with stressful moments without turning to violence.

His advice to parents at the end of their rope: "Step back, walk away, get help."